Polyesters have been developed conventionally as a material excellent in durability and heat resistance for fibers and plastic films. Nowadays, polyesters are also utilized for adhesives taking advantage of the above characteristics. Examples of such an adhesive include a hot melt type adhesive and a heat-curing type adhesive, which contains a curing agent and causes reaction with heat. On the other hand, pressure-sensitive adhesive sheet products having a pressure-sensitive adhesive as the base are utilized in many fields for joining (e.g., fixing of parts), for packing corrugated boards, etc. For the purposes of adhering well and retaining the property, these pressure-sensitive adhesive sheet products are designed to have a high tack (short-time adhesion), a high adhesive force, and a high cohesive force and also to adhere to any various complicated materials.
However, a conventional polyester series adhesive has a far higher melting point (softening point) than room temperature or is in a considerably highly crosslinked state for keeping and giving the adhesive property, and such an adhesive is very hard at about room temperature. Accordingly, it is difficult to utilize the conventional adhesive as a so-called pressure-sensitive adhesive which is required to have a tacking property at about room temperature, that is, a property capable of effecting adhesion by pressing for a short time or lightly.